This application claims the priority of Japanese Patent Application No. 4-335659 filed Nov. 19, 1992, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a secondary battery and, more specifically, to improvement of the binder used therefor in order to improve the cycle life and reliability (safety) thereof.
2. Description of the Prior Art
In recent years, there have been proposed, as negative electrode materials for lithium secondary batteries, carbon materials such as coke and graphite, in replacement of conventional metallic lithium, on the grounds of high flexibility and no danger of precipitating mossy lithium.
Negative electrodes utilizing the above carbon materials are generally prepared by, for example, a process which comprises making a slurry by dispersing a carbon powder (such as graphite or coke powder) and, as required, a conductive agent powder (such as acetylene black or carbon black) in a binder solution, applying the thus obtained slurry on a collector metal and drying the slurry on the metal.
As the binder solution, there has been principally used a solution of PVDF (polyvinylidene fluoride) in NMP (N-methyl-2-pyrrolidone).
However, although PVDF is excellent as a binder to consolidate carbon particles, it is poor in adhesiveness (close fittability) with collector metals. As a result, when charge-discharge is conducted repeatedly, the carbon powder used peels off from the collector metal such as copper plate or copper foil, thereby gradually decreasing battery capacity. That is, batteries utilizing PVDF have had the problem of more or less short cycle life.
Besides, when the battery temperature becomes abnormally high due to short-circuit or like accidents, PVDF decomposes to generate HF (hydrogen fluoride), which reacts vigorously exothermically with C.sub.6 Li having generated on the negative electrode during charge so that the battery may break or burst. Thus, the batteries utilizing PVDF have had problems with respect to reliability.